Manufacturing formulations involving aqueous dispersions of solid or liquid particulates often require to be processed at a high concentration for one or more of the following reasons: to improve the costs/efficiency of transportation, to increase laydown or to reduce the drying load, as the removal of water is wasteful and expensive. The formulations also need to be stable over a wide range of conditions. After mixing, aqueous dispersions of particulates, especially those containing high concentrations of solid, tend to form a system that is either highly viscous or behaves as a ‘yield stress material’ such as a gel, especially at low shear. The high viscosity or gel-like nature of such dispersions at low shear affects their dispersability and flowability, making the systems difficult to stir, pump, transport, coat or pour.
At first it is useful to reduce the viscosity of such dispersions to aid their initial processing and manipulation, for example in a coating, pumping or pouring process. Having attained the required geometry the next process is often one of drying where the objective is to ‘fix’ or ‘set’ the system in a solid state. The normal objective here is to maintain the uniformity attained at the completion of the manipulation stage (e.g. coating or pouring). Drying processes usually involve the application of heat and currents of air (and sometimes the control of humidity in aqueous systems). Air currents and machinery vibrations can perturb the uniformity of such systems, which is undesirable. Unfortunately dispersants used to reduce the viscosity of formulations to aid the wet processing of a product usually cause the formulation to decrease further in viscosity with increasing temperature, which makes the product more prone to perturbations until evaporative losses reverse this process and cause the viscosity to rise.
Thermally-responsive polymers, in which the solubility is dependent on temperature, are well known, the most common being those based on poly-ethyleneoxide (also known as PEO, polyethyleneglycol or PEG), poly(N-isopropyl-acrylamide) (also known as, and hereinafter identified as, poly-NIPAM) and poly-(N-vinylcaprolactam) (also known as PNVCap or PVCL). Polymers that exhibit a lower consulate solution temperature (LCST) or volume phase transition temperature (VPTT) would provide suitable materials to give thermally-responsive dispersants.
By far the most common application of these systems appears to be in the formation of hydrogels for controlled drug delivery. Thus Shin et al. in Journal of Applied Polymer Science (1997), 65(4), 685-693 disclose “Indomethacin-release behaviour from pH and thermo-responsive poly(vinyl alcohol) and poly(acrylic acid) interpenetrating network hydrogel for site-specific drug delivery”. Cammas et al. in Journal of Controlled Release (1997), 48(2, 3), 157-164 describe “Thermo-responsive polymer nanoparticles having a core-shell micelle structure as site-specific drug carriers”. Yang et al. describe “Thermoresponsive Gelatin/Monomethoxy Poly(Ethylene Glycol)-Poly(D,L-lactide) Hydrogels: Formulation, Characterization, and Antibacterial Drug Delivery”in Pharmaceutical Research (2006), 23(1), 205-214 and Kharlampieva et al. disclose “Hydrogen-Bonded Multilayers of Thermo-responsive Polymers” in Macromolecules (2005), 38(25), 10523-10531.
Another biochemical application of thermally-responsive polymers is in bio-separation processes wherein thermally-responsive hydrophobic chromatography can be utilised by using thermal gradients, as described by Kikuchi et al. in “Temperature-Responsive, Polymer-Modified Surfaces for Green Chromatography, ”Macromol. Symp., (2004), 207, 217-227. A further application for thermally-responsive polymer is in the cosmetics field.
However, examples of thermally-responsive, (also known as “thermo-responsive”, “temperature-dependent” or “thermally-switchable”) dispersants for positively- or negatively-charged or chargeable particles (for example inorganic oxides/hydroxides) in aqueous media are less well known.
WO 2003/004767 describes an aqueous slurry or dispersion of pigment particles comprising as dispersant an amphiphilic polymer at least partly adsorbed to the pigment particles, wherein the slurry or dispersion exhibits a temperature-dependent viscosity, such that the viscosity increases by a factor of at least about two when the temperature is raised from about 20° C. to about 100° C.
WO 98/56733 describes a method for manufacturing moulded ceramic and metallic products and crystallographically oriented anisotropic products from powders and powder dispersions in aqueous media, wherein the temperature of the fluid dispersion solids of high solids content is raised once it has been manipulated into its final shape. The surface of the particles is coated with a dispersant having a temperature-dependent solubility in the aqueous medium by the use of a dispersant comprising anchor and stabilising groups. The anchor group(s), which is preferably a silane group, anchors the dispersant to the surface and the stabilising groups, which are alkyleneoxides, are temperature-sensitive to the solvent medium. At low temperature the stabilising groups are solvated and the particles are spaced apart, whereas at higher temperatures the alkyleneoxides groups de-solvate and contract, leading to aggregation of the particles due to attractive forces.
WO 2006/067453 and WO 2006/067457 describe the use of an anchored stabilizer-form of dispersant for an aqueous composition (or dispersion) of positively-charged or chargeable solid particulates. However, they do not suggest the use of thermally-responsive polymers for the stabilizer part of the anchored dispersant. WO 2006/067457 teaches by way of comparative examples that the use of poly-ethoxylated blocks in the stabilizer/buoy part of a potential anchor-buoy form of dispersant is not suitable for dispersing an aluminium oxide dispersion at ambient temperatures.
WO 2007/071960 describes the use of an anchored stabilizer form of dispersant for an aqueous composition (or dispersion) of negatively-charged or chargeable solid particulates. However again there is no suggestion of the use of thermally-responsive polymers for the stabilizer part of the anchored dispersant.
Thus existing prior art thermally-responsive dispersants are    (a) thermally-responsive polymers/copolymers that are not designed with a terminal-anchoring group;    (b) anchored stabilisers with a terminal anchoring group and a stabilising group based on a thermally-responsive polymer polyalkyleneoxide; or    (c) anchored polymeric stabilisers with anchor groups based on an acid/OH or basic group wherein the polymeric stabiliser is not a thermally-responsive entity.
Owing to the above limitations, existing prior art materials are either ineffective or show limited efficiency/effectiveness as thermally-responsive dispersants, particularly for dispersions containing colloids with positively- or negatively-charged or chargeable oxide/hydroxide surfaces.